The mainstream of recent electrophotographic image forming apparatuses is a tandem type having a photosensitive member for each color to speed up printing. In the tandem-type image forming apparatus, for example, a detection image that is a developing material image used to detect a color misalignment or density is formed on an intermediate transfer belt. The color misalignment or density is corrected by detecting reflected light from the detection image using an optical sensor.
Japanese Patent Laid-Open No. 1991-209281 discloses providing two optical sensors that respectively detect specular-reflected light (to also be referred to as mirror-reflected light) and scatter-reflected light from a toner image and controlling the image density in accordance with the output difference between the two optical sensors. Japanese Patent Laid-Open No. 2003-76129 discloses an optical sensor that detects both specular-reflected light and scatter-reflected light using a prism. In these methods, one light-receiving element detects only the scatter-reflected light components, and correction is performed by, for example, subtracting the scatter-reflected light from the sum of the scatter-reflected light and specular-reflected light detected by the other light-receiving element, thereby extracting only the specular-reflected light components. In a method of detecting the density from the extracted specular-reflected light components, not the scatter-reflected light from the toner but the specular-reflected light from the background is mainly detected. Hence, the density can be detected independently of the color of the developing material that generates a difference in the scatter-reflected light amount. It is also supposedly possible to attain a high detection capability for a highlight region that is sensitive to the human visual characteristic. In the method of Japanese Patent Laid-Open No. 1991-209281, however, the error in correction processing of extracting only the specular-reflected light components becomes large. Japanese Patent Laid-Open No. 2005-300918 discloses reducing the effective spot diameter of specular-reflected light to lower the ratio of scatter-reflected light and thus improving the accuracy.
Consumption of the developing material by the detection image for color misalignment or density detection is required to be as low as possible. That is, the detection image is preferably made as small as possible. Even for a small detection image, a sensor having a high spatial resolution is necessary to accurately detect the density. Japanese Patent Laid-Open No. 2005-241933 discloses a sensor having a smaller irradiation area on the light emission side.
When the spot diameter of specular-reflected light is reduced in the conventional optical sensor, a variation of the LED chip position in the optical sensor or a mechanical variation of the converging mechanism greatly affects the yield in the manufacture or the detection accuracy. For example, to raise the spatial resolution of the optical sensor, the converging mechanism needs to be small. However, according to Japanese Patent Laid-Open No. 2005-241933, the spot diameter of the specular-reflected light is limited to about 1 mm when the variation in the manufacture and the like are taken into consideration. In addition, noise generated by the fine uneven pattern of the intermediate transfer belt surface becomes large as the spatial resolution of the optical sensor rises. As a result, the S/N ratio lowers, particularly affecting the density detection accuracy.